Hierarchical AssemblyDOI: 10.1002/anie.201210024

Simulation-Assisted Self-Assembly of Multicomponent Polymers intoHierarchical Assemblies with Varied Morphologies**Chunhua Cai, Yongliang Li, Jiaping Lin,* Liquan Wang, Shaoliang Lin, Xiao-Song Wang,* andTao Jiang

Self-assembly of two or more components simultaneouslyinto defined hierarchical structures is ubiquitous in nature.Understanding this natural phenomenon could enhance thepower of supramolecular chemistry for the development ofnovel functional materials.[1] Based on developed blockcopolymer supramolecular chemistry, solution self-assemblyof block copolymers with a second component, includingnanoparticles,[2] small molecules,[3] metallic and organometal-lic species,[4] and macromolecules,[5] has been experimentallyexplored and has led to a variety of interesting nanostructuresand functional materials.

Synthesis of defined nanostructures using concurrent self-assembly of multiple components is a great challenge, butmay offer more opportunities for the creation of novelnanostructures. A good example is the discovery of multi-compartment cylindrical micelles with phase-separated coresby co-assembly of a linear poly(acrylic acid)-b-poly(methylacrylate)-b-polystyrene triblock copolymer in the presence oforganic diamines.[3a] Stupp et al. investigated the self-assem-bly of peptide amphiphiles in the presence of dumbbellmolecules containing hydrophobic oligo(p-phenylene ethyl-ene) segments with bulky hydrophilic end caps, resultinguniform nonspherical supramolecular aggregates with thepeptide amphiphiles coated on the rigid-rod dumbbellmolecules.[5e] This early research developed a concept forpossible nanostructure design by self-assembling amphiphilicmolecules using molecular templates.

Very recently, we have discovered that poly(g-benzyl-l-glutamate)-b-poly(ethylene glycol) (PBLG-b-PEG) rod–coilblock copolymers and rigid PBLG homopolymers were ableto co-assemble into virus-like superhelical rods and rings withuniform diameter and screw pitch.[6] It is believed that thehierarchical structures were formed with PBLG bundles ascores wrapped by PBLG-b-PEG block copolymers. Obvi-ously, the structure of this system, as a result of the co-assembly of the two macromolecules, is quite different fromother previously reported helical structures self-assembledfrom single polymers.[3b, 7] It is therefore worth carrying outfurther studies to understand the system for possible designedsynthesis. However, because the system involves manypossible interactions between the two polymers, includinghydrophobic, dipolar p–p interactions and an ordered pack-ing tendency of a-helical polypeptide segments, it is a daunt-ing task for us to understand exactly how the observedstructures were formed. In fact, many other studies in the areaface the same challenge.

Computer simulation, such as Brownian dynamics (BD),has emerged as a powerful method for the investigation ofself-assembly behavior of amphiphilic block copolymers.Recent research has confirmed that simulations are capableof capturing the essential feature of the experimental findings,including multicomponent complex self-assembly systems.[8]

What particularly important is that the simulation can providemicroscopic information about the self-assembled aggregates,which could not to be discerned using currently availableexperimental techniques. Combining the experimental andsimulation work is therefore an effective strategy for theinvestigation of polymer self-assembly.[8g]

Herein, we present the Brownian dynamics (BD) simu-lation-assisted, designed self-assembly of a binary systemcontaining rod–coil (or coil–coil) block copolymers and rod-like (or coil) homopolymers. The simulation results suggestthat a number of parameters, including interpolymer associ-ation forces, rigidity of polymers, and mixture ratio of the twopolymers, influence the self-assembly behavior. By adjustingthese parameters, the morphologies of the self-assembledobjects can be systematically varied from abacus-like (beads-on-wire) structures to superhelices and plain fibers. Instructedby these simulation results, we have synthesized an array ofblock copolymers (for example PBLG-b-PEG and PS-b-PEG) and homopolymers (such as PBLG and PS), fromwhich a number of binary systems were composed, allowingthe simulated parameters to be investigated experimentally.The results follow the simulation predictions, allowing usunderstand the assembling behavior of the investigated

[*] C. Cai, Y. Li, Prof. J. Lin, L. Wang, S. Lin, T. JiangShanghai Key Laboratory of Advanced Polymeric MaterialsState Key Laboratory of Bioreactor EngineeringKey Laboratory for Ultrafine Materials of Ministry of EducationSchool of Materials Science and EngineeringEast China University of Science and TechnologyShanghai 200237 (China)E-mail: jlin@ecust.edu.cn

Dr. X.-S. WangDepartment of ChemistryWaterloo Institute of Nanotechnology (WIN)University of Waterloo, Waterloo, N2L 3G1 (Canada)E-mail: xiaosong.wang@uwaterloo.ca

[**] This work was supported by the National Natural ScienceFoundation of China (50925308 and 21234002), Key Grant Projectof the Ministry of Education (313020), and the National BasicResearch Program of China (No. 2012CB933600). Support fromprojects of Shanghai municipality (10GG15 and 12ZR1442500) isalso appreciated. X.-S.W. thanks the NSERC and the University ofWaterloo for financial support.

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/anie.201210024.

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system and synthesize hierarchical supramolecular polypep-tide nanostructures in a predictable fashion.

The simulation system contains rigid homopolymers A150

and rod–coil block copolymers A7B3, where the rigid Asegments are solvophobic and the coil B segments aresolvophilic. The coarse-grained molecular dynamics program(COGNAC) of OCTA was employed for the simulations.[9]

During the simulation, the amphiphilicity existed in themulticomponent systems was defined by setting differentinteraction potentials. The interactions between A and A (A–A interactions), which is considered as a major parameterdominating the cooperative self-assembly, were modeled witha potential containing an attractive component. Such selec-tion makes A blocks perform as a solvophobic part for theformation of the cores of the aggregates. The interactionsbetween B and B blocks (B–B interactions) were modeledwith a purely repulsive potential, thus accounting for a sol-vophilic nature of the block. Furthermore, the incompatibilitybetween A and B blocks was also modeled with a purelyrepulsive potential. The simulation details, including choicesof parameters, can be found in the Supporting Information.

The strength of A–A interactions (eAA) was varied from2.4 to 2.1 to 1.6. The effect of eAA on the simulation results arepresented in Figure 1a–c; three distinctive hierarchical mor-

phologies with A7B3 coating on A150 bundles are revealed. Ascan be seen, the block copolymers and homopolymerscooperatively associate into a large-length-scale fiber struc-ture with the block copolymers forming small-length-scalestructure on the homopolymer bundles. When eAA is higher(2.4), an abacus-like structure (beads-on-wire) is observedwith A7B3 aggregated into separating beads around A150

bundles (Figure 1a). As eAA decreases to 2.1, A7B3 becomehelically wrapping on the A150 bundles (Figure 1b). While

further decreasing the eAA to 1.6, full integration of the twocomponents cannot be achieved. As shown in Figure 1c,although the bundles of A150 are randomly coated with A7B3,many A7B3 chains are freely dispersed in the simulation cell.This set of simulation emphasizes the important role ofinteraction strength of the rod blocks for the synthesis andvariation of hierarchical nanostructures. Furthermore, themixture ratio of A7B3 to A150 has a prominent effect on theformed hierarchical structures (see the Supporting Informa-tion). For example, by decreasing molar ratio (MR) of A7B3 toA150 at the condition eAA = 2.1, the helical structure becomesgradually less apparent owing to less A7B3 coated on the A150

axis (Supporting Information, Figure S1). The simulationsalso revealed that a long rigid homopolymer is essential forthe formation of hierarchical structures. Mixture systems withshorter rigid homopolymers self-assembled into spheres orshort helical rods (Supporting Information, Figure S2).

To understand the importance of the rigidity of A block inthe assembly, coil–coil block copolymers of C7B3 were used toreplace the rod–coil block copolymers of A7B3 for the study.The interaction parameter between A and C (eAC = 2.1) wasset to be equal to eAA and eCC. As shown in Figure 1d, despitethe high ratio of C7B3 with respect to A150 that was used, thesimulated assembling only leads to plain fibers with C7B3

smoothly coating on the surfaces of A150 bundles. IncreasingeAC does not change the morphology obviously (SupportingInformation, Figure S3). These simulation results stress thepoint that the rigidity of the polymer blocks is essential indetermining the fine structure of the assemblies.

As we learned from the simulation results, self-assemblyof the binary systems can be influenced markedly by theinteraction strength of hydrophobic blocks. To experimentallyexplore such a parameter, a set of experiments on the samepair of polymers (PBLG-b-PEG/PBLG) at various temper-atures was designed. It is well-known that PEG is water-soluble and its solubility decreases with increasing solutiontemperature.[10] Therefore, at higher temperatures, the blockcopolymers are less hydrophilic, which induces a relativelystronger attraction between hydrophobic PBLG segments; atlower temperatures, increased hydrophilicity of PEG seg-ments in the block copolymers renders the interactionbetween PBLG segments relatively weaker. Indeed, a drasticinfluence of solution temperature on the self-assembledstructures has been observed.

Self-assembly of the PBLG31000-b-PEG5000/PBLG528000

binary system (the subscripts denote the molecular weightfor each segments) was first performed at 20 8C. The weightratio of block copolymer to homopolymer is 4:1 (whenconverted into molar ratio, it is ca. 58). Figure 2a is a TEMimage of self-assembled structures. As can be seen, uniformsuperhelical structures, including fibers and rings, are clearlyobserved, which is consistent with our previous report.[6] Thesupramolecular structures have PEG-rich corona, as deter-mined by 1H NMR analysis (Supporting Information, Fig-ure S4). The inset in Figure 2a is the simulation prediction ofthe superhelical structure in which homopolymer bundlesform the inner axis and block copolymers form screwsthrough ordered packing of the rod segments. The exper-imental results combined with simulation predictions indicate

Figure 1. Simulation predictions for hierarchical structures self-assem-bled from rod–coil block copolymer/rigid homopolymers (A7B3/A150):a) With higher interactions, eAA = 2.4; b) with moderate interactions,eAA = 2.1; c) with lower interactions, eAA = 1.6, and d) self-assembledfrom coil–coil block copolymers/rigid homopolymer (C7B3/A150),eAA = eAC = eCC = 2.1. The molar ratio of block copolymer to homopoly-mer is 660. The blue, green, red and olivine beads correspond to thehomopolymers A, A block, B block, and C block of block copolymers,respectively.

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that the superhelical structure is formed from hydrophobicPBLG homopolymer inner bundles wrapped by the PBLGrod segments from the PBLG-b-PEG block copolymers. Thesuperhelical feature of the aggregates is confirmed using cryo-TEM (Figure 2b). As the cryo-TEM reveals the samemorphology as conventional TEM testing, possible dryingeffect for the formation of observed helical structure can beruled out. The diameter and screw-pitch of the superhelices,as estimated from the cryo-TEM image (Figure 2b), areabout 140 and 95 nm, respectively. The self-assembledstructure was further examined using SEM (Figure 2c) andAFM (Figure 2 d), from which three-dimensional shapes ofsuperhelical fibers and rings are clearly observed. Moreover,the images show that all these helices are right-handed.

In a separate experiment, we have examined the influenceof chirality of polypeptides by replacing PBLG with poly(g-benzyl-d-glutamate) segments (PBDG; takes opposite chir-ality to PBLG). Right-handed superhelices were alsoobserved from PBDG-b-PEG/PBDG mixtures. However,when the handedness of polypeptides in the block copolymerand homopolymer is opposite, such as PBLG-b-PEG/PBDGand PBDG-b-PEG/PBLG mixtures, the helical structuresbecome less regular (Supporting Information, Figure S5).These results suggest that the same handedness of polypep-tides in block copolymer and homopolymer is essential for theformation of ordered hierarchical structures.

To investigate the effect of self-assembling temperature,the same experiments were further carried out at lowertemperatures of 5 and 10 8C, and higher temperatures of 30and 40 8C. Figure 3a,b shows SEM images of the aggregatesprepared at lower temperatures. As shown in Figure 3a, fibers

with rough surface or irregular helical sense are observedwhen the preparation temperature is 10 8C. For the samplesprepared at 5 8C, plain fibers are obtained (Figure 3 b).Figure 3c,d shows the SEM images of the aggregates pre-pared at higher temperatures: a new morphology appears,namely an abacus-like structure. For the sample prepared at30 8C, the abacus-like structures coexist with helical structures(Figure 3c). Upon increasing the experimental temperatureto 40 8C, the abacus-like structures become the predominantmorphology (Figure 3d). As indicated by simulated structures(see Figure 1a and the inset in Figure 3d), the abacus-likestructures are formed with block copolymers aggregating intoseparated beads around the homopolymer bundles. Thesenanostructures are also confirmed using TEM (SupportingInformation, Figure S6) and Cryo-TEM (Supporting Infor-mation, Figure S7).

Apart from temperature, the effects of other factors onthe self-assembled structures were also briefly investigated.The helical feature becomes less apparent upon decreasingthe mixture molar ratio (MR) of block copolymer tohomopolymer from 58 to 14. When further decreasing theMR to 6, fibers with smooth surface were obtained (Support-ing Information, Figure S10). These results are in agreementwith the simulation results (Supporting Information, Fig-ure S1). Moreover, a relatively higher molecular weight ofPBLG homopolymer is essential to obtain a helical structure(Supporting Information, Figure S11); for PBLG-b-PEGblock copolymers, both the PBLG and PEG block lengthsshould be moderate (Supporting Information, Fig-ures S12, S13). Finally, the initial polymer concentration inorganic solvents is found to have some influence on the self-assembled structures. Similar superhelical structures have

Figure 2. a) HRTEM, b) cryo-TEM, c) SEM, and d) AFM images ofsuperhelices prepared at 20 8C. MR=58. Inset in (a): simulationprediction of the superhelical structure. Scale bars: 500 nm (a,c),100 nm (b). Samples for (a, c, and d) were prepared from aqueoussolution; the sample for (b) was prepared from vitrification of aqueoussolution using liquid ethane.

Figure 3. SEM images of aggregates self-assembled from the PBLG-b-PEG/PBLG binary system at various temperatures: a) 10 8C, b) 5 8C,c) 30 8C, and d) 40 8C. Inset in (d): simulation prediction of theabacus-like structure. Scale bars: 200 nm. Samples were preparedfrom aqueous solution.

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been observed with the initial polymer concentrations rangingfrom 0.05 to 0.9 gL�1. However, upon increasing the initialpolymer concentrations, the diameter increases, and the screwof the helices becomes relatively tighter (Supporting Infor-mation, Figure S14).

Based on simulation and experimental results, the supra-molecular morphology variation from plain fiber to super-helix and then to abacus-like morphologies can be attributedto gradually increased PBLG–PBLG interactions as a func-tion of temperature. The strong association of PBLG–PBLGfacilitated an ordered packing (side-by-side) of PBLG rods onthe surface of PBLG bundles, leading to abacus-like mor-phologies (Figure 3 d). As estimated from Figure 3d, thewidth of ridge regions in the abacus is about 30 nm, which isonly slightly larger than the length of PBLG rods (ca.22 nm),[11] indicating that PBLG rods are packed in a rela-tively highly ordered fashion. By decreasing the interactionsbetween PBLG–PBLG (experimentally achieved by drop-ping self-assembly temperatures), the packing order of PBLGrods decreased and gradually transform from side-by-side todislocated and to random fashion (Figure 1). Consequently,the widths of ridge assembled from PBLG blocks graduallyincreases from 30 nm for abacus-like structure to 35 nm forhelix structure, and finally become indistinguishable in theplain fibers (Figure 3b) as a result of random packing ofPBLG-b-PEG block copolymer on the surface of PBLGhomopolymer bundles. Detailed discussions of the mecha-nism for the formation of these abacus-like structures,superhelices, and plain fibers are given in the SupportingInformation, Section S2.14.

To experimentally explore the effect of rigidity of hydro-phobic segments on the self-assembled structures, we pre-pared polystyrene-b-poly(ethylene glycol) (PS-b-PEG) coil–coil block copolymers and PS homopolymers for our inves-tigation. In contrast to PBLG, PS is a hydrophobic coilpolymer. The interaction between PBLG and PS segmentswas considered to be comparable to that between PBLG andPBLG because both hydrophobic interaction and p–p stack-ing of phenyl groups existed in both PBLG–PS and PS–PSpairs.[12] Through selectively pairing up the block copolymersand homopolymers, the effect of rigidity of polymer chains onself-assembly behavior of the binary systems can be eval-uated.

First, PS-b-PEG was used to self-assemble with PBLGhomopolymers at 20 8C. TEM image reveals plain fibers andrings without helical or abacus-like features (Figure 4a). Thisresult is consistent with simulated structures (Figure 1 d),emphasizing the importance of rod–coil block copolymersrequired for small-length-scale features (helical or abacus-like). Furthermore, we do not observe the effect of temper-ature on the self-assembled morphology for such a system.These control experiments verified the theoretical results thatthe rigidity of hydrophobic segment in block copolymer isnecessary for the formation of ordered packing of the blockcopolymer on the surface of PBLG bundles.

To test the importance of rigidity of core-forminghomopolymers, the self-assembly of the PBLG-b-PEG/PSbinary system was investigated. Although we could notsimulate the system owing to the limitation of current

simulation technique,[13] experimental studies follow ourintuition that spherical rather than one-dimensional struc-tures are obtained as a result of using PS homopolymers.Moreover, we have discovered a new morphology that is likethat of a ball of wool. As shown in the SEM image(Figure 4b), spheres with stripes on the surface are observedfrom the system assembled at 20 8C. Based on the knowledgedeveloped from above investigation on superhelical andabacus-like structures, it is possible that the balls of woolconsist of PS homopolymers cores covered by PBLG-b-PEGblock copolymers. Owing to the rigidity of PBLG, the blockcopolymers are able to pack into ordered structures, leadingto the formation of the stripes on the surface of the balls. Totest this hypothesis, we performed the same experiment but athigher and lower temperatures, respectively. At the highertemperature (40 8C), larger spheres with regular stripes wereobtained (Figure 4 c). However, the lower temperature (5 8C)leads to the formation of smooth spheres without stripe-patterned surfaces (Figure 4d). Obviously, the morphologyvarying from balls of wool to smooth spheres can be explainedby a decrease in association forces between the blockcopolymers at lower temperature as we discussed above.

In conclusion, with the assistance of computer simulation,we systematically investigated the self-assembling behavior ofblock copolymer/homopolymer binary systems, leading toa better understanding of supramolecular chemistry on suchsystems. As a result, we were able to rationally synthesizea number of hierarchical superstructures, including fibers,helices, abacus-like structures, spheres, and balls of wool, byvarying a few parameters, such as temperature, ratio of thetwo polymers, and rigidity of polymer blocks. The work hasdemonstrated the possibility to vary nanostructures of multi-

Figure 4. Images of aggregates self-assembled from: a) PS-b-PEG/PBLG, 20 8C, HRTEM; b) PBLG-b-PEG/PS, 20 8C, SEM; c) PBLG-b-PEG/PS, 40 8C, SEM; d) PBLG-b-PEG/PS, 5 8C, SEM. The weight ratio ofblock copolymer to homopolymer is 4:1 (a); 1.5:1 (b–d). Scale bars:500 nm. Samples were prepared from aqueous solution.

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component assemblies in a designed fashion and to createnovel morphologies. Furthermore, the obtained hierarchicalsuperstructures resemble viruses in structure, which consistsof proteins spontaneously assembled around a DNA/RNAtemplate. The information gained from the present work mayprovide useful guidance for construction of model viruses andsubsequently facilitate the investigation of the physiologicalbehavior of viruses, for example, cell penetration.

Experimental SectionTo prepare self-assemblies, block copolymers of PBLG-b-PEG andPS-b-PEG and homopolymers of PBLG and PS were first dissolved ina tetrahydrofuran (THF)/N,N’-dimethylformamide (DMF) (3:7 v/v)solvent mixture. The polymer concentration of the stock solutions wastypically 0.3 gL�1. Block-copolymer and homopolymer solutionswere first mixed with the intended volume ratio, typically 8 mL ofPBLG31000-b-PEG5000 block copolymer solution and 2 mL ofPBLG528000 homopolymer solution. For the study of the effect of themixing ratio of block copolymer to homopolymer on the self-assembly, the initial total polymer concentrations are fixed at0.3 gL�1, and the feeding ratio of block copolymer to homopolymerwas varied for the study. Deionized water (2.5 mL), a selective solventfor PEG, was added to the system at a rate of ca. 1 mLmin�1 withvigorous stirring. Upon the addition of water, the colorless solutiongradually took on a blue tint, which indicates the formation of self-assembled aggregates. Finally, the solution was dialyzed againstdeionized water for at least 3 days to remove organic solvents.

All experimental procedures, including the processes of addingwater and dialysis, were performed at a constant temperature. Thepolymer solutions, water for micellization, and water for dialysis werestored at a corresponding temperature for more than 12 h before use.The self-assembling experiments were conducted at temperatures of5, 10, 20, 30, or 40 8C. The aggregate morphologies were characterizedby TEM (JEM-2100F, 200 kV), Cryo-TEM (JEM-2200FS, 200 kV,�174 8C), SEM (S4800, 10 kV), and AFM (XE-100, tapping mode).Detailed experimental information is available in the SupportingInformation.

The self-assembled structures are equilibrium under the initialself-assembly conditions (water/organic solvent = 1:5 v/v). The aggre-gates of superhelices and abacus-like structures are reversible whentemperature varies; for example, the superhelices transform toabacus-like structures when the temperature is increased, and theabacus-like can be reversibly transformed back to the superhelices bydecreasing the temperature (Supporting Information, Figure S8).After dialysis against water, the hierarchical structures are frozen.The store temperature has negligible effect on the morphologies ofthe frozen aggregates. For example, no morphology transformationwas observed even the aggregates were aged at various temperatureup to 40 8C for more than 3 months (Supporting Information,Figure S9).

Received: December 15, 2012Revised: April 3, 2013Published online: June 17, 2013

.Keywords: block copolymers · hierarchical assembly ·homopolymers · polypeptides

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